Japanese Patent Application Publication No. 2012-80221 discloses a target tracking device that, in a case where a tracking target is screened by a plurality of screening objects, sets, as an object to be tracked, the one of the screening objects with the longest estimated screening duration calculated based on the difference between the motion vector of the screening object and the motion vector of the tracking target.
However, in the technique described in Japanese Patent Application Publication No. 2012-80221, the speed of the tracking target at the point when the tracking target was screened is used as its speed. For this reason, in a case of using the technique described in Japanese Patent Application Publication No. 2012-80221 to estimate the inter-vehicle distances in a line of vehicles including a tracking target, a change in the speed of the tracking target may possibly decrease the accuracy of the inter-vehicle distance estimation.